The present invention relates generally to adaptive quantization and, more particularly, to an adaptive quantization method and device for an orthogonal frequency division multiplexing (“OFDM”) receiver.
Orthogonal frequency division multiplexing (“OFDM”) and punctured convolutional codes have been used in digital wireless communication systems, such as those defined by the wireless local area networks (“WLAN”) standard, IEEE 802.11g, to provide bandwidth-efficient wireless communications and reduce signal distortion caused by multi-path propagation. FIG. 1 is a block diagram of a conventional wireless communications system. Referring to FIG. 1, the wireless communications system includes a transmitter 10, a channel 20 and a receiver 30. The transmitter 10 further includes a convolutional encoder 11, a puncturer 12, and an interleaver 13, which arranges its output into groups of ns bits and feeds them into a sub-carrier quadrature amplitude modulation (“QAM”) mapper 14. The number of bits ns depends on the interleaving scheme, which may include one of BPSK (ns=1), QPSK (ns=2), 16-QAM (ns=4), or 64-QAM (ns=6). The sub-carrier QAM mapper 14 maps an OFDM symbol into multiple sub-carriers. For each of the sub-carrier QAM symbols, an OFDM modulator 15 performs an Inverse Fast Fourier Transform (“IFFT”) to generate an OFDM symbol in the baseband, with each OFDM symbol being represented by nOFDM baseband samples. A PHY burst 16 receives these nOFDM data samples and adds the ncp cyclic prefix samples. The PHY Burst 16 also performs a windowing function and inserts before the first nOFDM+ncp OFDM data samples the following baseband signaling samples: (1) a few short pre-amble OFDM symbols, (2) a few long pre-amble OFDM symbols, and (3) a signaling OFDM symbol in sequence. The cyclic prefix for each OFDM symbol provides a guard time for multi-path mitigation at a receiver. The windowing function serves to lower the side-lobes of the transmit spectrum and hence helps minimize adjacent channel interference. The short pre-amble symbols are used for packet detection, automatic gain control, and coarse frequency estimation at the receiver. The long pre-amble symbols are used for fine frequency estimation and channel estimation at the receiver. The signaling OFDM symbol contains information such as the sub-carrier modulation and coding scheme (“MCS”) required for receiver operation. Next, an RF transmitter 17 receives the output of the PHY burst 16 and performs all transmitter functions such as digital-to-analog conversion, filtering, up-conversion, amplification, and radiation into the air. The output of the RF transmitter 17, an analog waveform, is transmitted over a channel 20, typically a “multi-path propagation” channel, and received at the receiver 30.
At the receiver 30, an acquisition and tracking 32 first uses the received base-band signal samples from the RF receiver 31 to detect the OFDM pre-amble and estimates the OFDM symbol boundaries. A Useful Data device 33 takes a block of the received base-band signal samples corresponding to one received OFDM symbol, removes the samples for the cyclic prefix and outputs only the useful nOFDM samples. An OFDM demodulator 34 takes one block of nOFDM samples at a time and performs a Fast Fourier Transform (“FFT”) to recover the m sub-carrier QAM symbols. A channel estimator 35 takes the FFT outputs for the long pre-ambles from the OFDM demodulator 34 and estimates the sub-carrier channel frequency responses (“CFR”), Gch(i), where i=0, 1, . . . , m−1, for all m sub-carrier channels. A QAM demapper 36 generates a total of m·ns soft-bits as outputs for each OFDM symbol, with ns soft-bits for each sub-carrier QAM symbol and each soft-bit containing the information required for Viterbi decoding.
These output m·ns soft-bits are “de-interleaved” by a de-interleaver 37 to recover their order and then delivered to the a de-puncturer 38, where the “punctured” bits are inserted back. The outputs from de-puncturer 38 are sent to a convolutional decoder 39, which performs an optimum decoding using the well-known Viterbi decoding algorithm and outputs decoded user information.
For wireless communications utilizing OFDM modulation, each QAM symbol can contain up to 6 bits with 64-QAM modulated sub-carriers. In addition, the sub-carrier channel frequency responses, Gch(i), can vary in amount by 20 dB due to multi-path fading. Therefore, additional 11 to 13 bits may be required to represent each of the sub-carrier QAM demapper output. To process signals with such a large dynamic range, complex hardware is required in downstream processors such as the de-interleaver 37, depuncturer 38 and decoder 39. It is desirable to have a simplified receiver design which can reduce the complexity caused by different modulation coding schemes and variations among sub-carriers.